Induction logging tools propagate a signal through strata adjacent to a well borehole to determine measurements relating to the rock formations making up the strata. Particularly important measurements are directed to bulk resistivity and the dielectric constant of the material. These measurements of the rock making up the formations indicate the presence of hydrocarbons found in the pore spaces of the rock formations. In particular, the present invention applies to the special case where the bulk resistivity alone is inadequate to determine in situ hydrocarbon saturation. This occurs when the connate waters in a given region are known to be very pure and have high resistivity, and are thus not readily distinguishable from non-conductive hydrocarbons. Investigating the dielectric properties where water has a dielectric constant at least twenty times that of hydrocarbons is involved. When a well is drilled, conventional drilling techniques involve the use of drilling mud which invades the formations and which may change the measurements. Conventional drilling techniques involve delivery of a continuous stream of drilling mud through the drill stem which flows upwardly in the annular space surrounding the drill pipe. The drilling fluid is made of two major components, one being a weight material, usually barite, and the other is water. As the well progresses in depth, ambient pressure within the unaltered formation is potentially interrupted by the borehole formed through the formation. It is possible that the fluid otherwise held in the formation at ambient pressures will escape into the borehole. To counter this, the column of drilling fluid in the borehole is maintained at a specified pressure to prevent discharge up through the borehole. If that were to occur, a "blowout" might well happen. The ambient pressure is thus counter balanced and slightly exceeded in optimum conditions by the pressure of the drilling fluid. That pressure is determined by two factors which are primarily the height of the column of drilling fluid in the borehole and the density of that fluid.
This positive pressure applied in the drilling column provides fluid drive forcing a portion of the drilling mud into the adjacent formations and the rock formations are invaded at least to some measure. The solvent will typically penetrate further into the formation than the solid components, thereby forming a mud cake on the sidewall of the borehole. Generally, this is desirable to protect the borehole. The depth of penetration of the drilling mud solvent however is variable and has a substantial impact on formation characteristics as a result of the invasion.
The penetration of the drilling fluid solvent is dependent on many factors including the nature of the rock formation, the pressure differential, the thickness of the mud cake, the wettability of the formation to the hydrocarbons and to the water based drilling fluids, and many other factors which are too complex to readily quantify. Thus, when an inductive measurement is made, the data obtained is substantially dependent on the depth of invasion into the formation. The present procedure enables determination of the depth of invasion, hence resistivity and dielectric measurements can be isolated for the unaltered formation, while the region which has been penetrated by the drilling fluid can also be separately evaluated.
The response of the formation is partially dependent on the frequency used to obtain the induction measurements. Heretofore, conflicting factors have forced a compromise in such measurements. The frequency ideally is sufficiently high to provide a measurable phase shift in response to dielectric constant changes. The frequency is ideally low enough to avoid skin effect limitations on the depth of investigation. Literature regarding this has described a generally acceptable frequency range of about 16 to about 200 Megahertz. The present system is directed to a plurality of measurement which are made at plural frequencies. It has been determined that the frequency range useful for accurate dielectric measurements accomplished with relatively deep inductive penetration into the formation is in the range of about 10 to about 200 megahertz. Such measurements are unreliable until the invaded zone resistivity measurement can be determined and separated from the data. By contrast to the range mentioned above, relatively low frequencies are preferred to obtain an appropriate phase shift in measuring the resistivity. This range is about 1 to about 10 megahertz. This is an entirely different range than the range involved in dielectric constant measurements. As set forth in greater detail, a plurality of measurements at selected frequencies is used to obtained different readings. This helps isolate the invaded zone so that the constrast in resistivity measurements can be found. If the resistivity contrast defining the invaded zone can be determined, then measurements become more meaningful so that the actual resistivity of the rock can be measured. Once that is known, higher frequency induction measurements are used to determine the dielectric constant.
The present approach thus involves sweeping the logging tool transmitters through a range of frequencies. The low frequencies are used in resistivity measurements, and in particular to locate the invaded zone. This enables the bulk resistivity of the rock to be isolated. That value is helpful in the remaining sequence. The sequence involves high frequency measurements which are useful in dielectric constant evaluation. Several measurements are made at higher frequencies, and that data will yield measured phase shift and attenuation. This data enables determination of the dielectric constant of the formation, and in particular, dielectric constant without invasion of the drilling fluid.
The present apparatus is thus summarized as a logging system having multiple transmitters and receivers along a sonde. These are pulsed at selected frequencies. The frequencies sweep across the low frequency range to assist in resistivity measurements and then across the high frequency range to obtain data useful in dielectric constant measurements. This requires the sonde to support a number of antenna elements where the several antennas must have a specified radiation pattern. For a formation of sufficient thickness to be resolved by the present procedure, the antenna propagation pattern must be considered.
First, notice should be taken of the context in which the antennas are placed in the borehole. They are mounted in an elongate sonde typically supported by a elongate cylindrical housing and typically having an elongate cylindrical mandrel therein. This in turn is located in an elongate cylindrical hole in a formation which has a interface at the borehole with or without borehole fluid invasion. Ideally, all of this has a common central axis. It is desirable that the electromagnetic energy traveling from transmitter to receiver have the form of a substantially planar wave traveling parallel to the axis of the borehole.
In a practical sense, parasitic electric fields are something of a problem. In part, this derives from the installation of multiple antennas on the mandrel making up the sonde. In part, this involves the coaxial transmission line which is defined by the mandrel surrounded by borehole fluids, the outer wall of the borehole, and the more remote surrounding formation defined by the invaded zone in the formation. The magnetic field of this defined coaxial line is orthogonally positioned to the desired propagation mode. It forms a wave traveling with a difference velocity and subject to different attenuation characteristics.
The desired wave in a perfect system involves an azimuthal electric field with a magnetic field at right angles, while the parasitic system has the magnetic field in the azimuthal direction and the electric field in the axial direction. The foregoing describes the unwanted transmitted signal; clearly, the receiving antenna may respond to it and thereby form an erroneous signal component.
The foregoing describes the problem somewhat simplistically; in fact, the installed antennas on the sonde (including the geometry of the sonde) positioned in the borehole prevents direct data separation; that is, the response in the unwanted mode is sufficiently significant and yet significantly complex that it cannot be sorted from the desired data and it is therefore very helpful to supress this unwanted signal mode.
Keeping in view that the antenna system must operate in the approximate range of about 1 to about 200 megahertz, the present disclosure is directed to an antenna system known as the shielded loop system. That is, it is formed of shielded conductors of the sort typically used in a transmission line. Multiple turns are configured so that the shielded coaxial cable eliminates any external electric dipole from being formed along the axis of a multiple turn coil shaped like a solenoid. All the turns are connected to the same potential by a shorting bar parallel to the axis of the coil. Multiple turns in the solenoid coil improve antenna gain while the band width of the antenna is determined primarily by the single turn inductance loaded by the transmission line characteristics impedance. If high impedance coaxial cable is used, the upper frequency limit of the antenna system is increased. This provides an induction coil coupled to a transmission line. While the foregoing described the transmitter antenna, an identical receiving antenna may be utilized, all subject to the principal of reciprocity.
While the foregoing speaks of a single transmitter antenna and a reciprocally defined receiver antenna, the present system contemplates the use of several antennas within the sonde which are all supported by a central mandrel through the sonde. The shorting bar mentioned above in the multiple turn shielded loop antenna. A solenoid coil provides a uniform mean surface potential. Two or more such solenoid coils can be used if they are attached to a common metallic mandrel which provides zero potential coupling. The mandrel supports all of the coils (more than two in the preferred embodiment) and thus the mandrel is involved in the shorting bar construction. By defining sets of transmitting and receiving antennas and locating them at various points along the mandrel, all undesirable interaction along the length of the sonde is eliminated. In this regard, the central metal member defining the mandrel may be used to support the plural antenna elements with spaced resistance material disks extending outwardly from the mandrel and which contact the tubular sleeve or housing which defines the sonde. That housing of course is in electrical contact with the surrounding borehole fluid. Transverse resistive disks thus extending from the central mandrel and contacting the surrounding resistive tubular sleeve or housing markedly decreases parasitically induced current flow resulting from the coaxial transmission line implemented by the central mandrel. A resistive housing is obtained by a composite plastic material such as an epoxy impregnated, glass fiber wrapped housing provided with sufficient fibers of conductive carbon or graphite or otherwise provided with conductive carbon particles in the epoxy. The composite material housing is made with a selected electrical conductivity to thereby avoid or at least substantially reduce the parasitically induced, undesirable currents in a way that is essential to a practical tool. Some of the transverse resistive discs may be replaced with highly conductive metal discs where appropriate to provide enhanced suppression of parasitic currents that may flow in the central mandrel. The housing may also incorporate conductive fibers that are oriented to enhance the electric field shielding of coils while not interfering with the desired magnetic field coupling of the coils.
In the preferred embodiment, there will typically be six antenna elements arranged at specified lengths along the structure of the tool and these will therefore operate quite successfully when installed on a mandrel in a housing as described with resistive transverse elements spaced along the length of the housing in the described fashion. This defines a multifrequency dielectric logging tool which operates at a wide frequency range, typically up to about 200 megahertz, which is pulsed periodically at selected frequencies and which provides the data described above. While some suggestion has been made above of the preferred embodiment, the detailed description which is set below will provide description of the tool of the present disclosure and will further provide a basis for a detailed explanation for its construction and operation. An appropriate, the underlying theory relating to its operation will also be given.